The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks on to this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time. The Russian operated global navigation satellite system (GLONASS) and the European Galileo positioning system are the two other important satellite-based navigational systems.
The GPS receivers can operate in many modes. In a “hot start” mode, the receiver already has stored in its memory the time, its last position, and the information on satellite position (also known in the art as almanacs and ephemeris). The receiver can use this stored information to determine which satellites are probably visible, and it can then lock on to those satellite signals in a short time. On the other hand, the receiver may have no prior data on its position, time, or almanacs stored. In this “cold start” mode, the receiver has to search for signals from all of the satellites present in the constellation. There are some other modes where partial information on time, position and almanacs are available and the corresponding start mode is known as “warm start.”
The GPS receiver has to acquire and lock on to at least four satellites in order to derive the position and time. Usually, a GPS receiver has many parallel channels, each receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random number (PN) code phase. Each satellite transmits signals using a unique 1023-chip long PN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PN code sequence to correlate with the digitized received satellite signal sequence. During acquisition stage, the code phase search step is a half-chip for most satellite navigational signal receivers. So the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
Once the satellite signal has been acquired, the receiver continues tracking the signal. At the same time, the receiver extracts the modulated navigation data from the signal being tracked. This tracking of the signal involves tracking the change in replica frequency relative to the received signal. This includes adapting the local replica frequency to the Doppler variation of the received signal. This Doppler frequency continuously changes with time as a function of the satellite position and receiver velocity. Under some conditions, such as traveling in a tunnel, the input signal may be blocked for a length of time. Consequently, the receiver may lose signal tracking for a brief period. When the satellites are visible again, the receiver starts reacquiring the satellite signal. This reacquisition time, however, can be much shorter than the time required for initial acquisition of the signal. After such a brief lapse, the search space in both frequency and code phase domain is smaller because the position, time and satellites information (such as ephemeris, almanac, etc.) may be assumed to be close to their previous values. Based on this information the carrier frequency and code phase can be precisely predicted. However, the correct code phase may differ from the predicted value by a small number of chips, depending on the duration of the signal block-out period. The corresponding frequency deviation is also small. The process of acquiring the signal in this case is known as reacquisition. The signal search range of reacquisition depends upon the duration of the signal loss and receiver dynamics.
It should be noted that during the reacquisition process, especially for high sensitivity receivers, the reacquisition might be a false reacquisition due to a lock on to the correlation side-lobes (false PN code phase) or to frequency side lobes (false carrier frequency). In this case, the bit synchronization and hence the navigation data extraction may not be possible, or incorrect data extraction may result with an associated large bit error rate.
The prior art has focused primarily on navigational signal acquisition techniques under various signal power conditions rather than techniques that address the specific problems associated with reacquisition. The U.S. Pat. No. 6,643,320 teaches that the expected signal power level is set to the level prior to the interruption. The U.S. Pat. No. 6,480,150 uses hardware for tracking. U.S. patent application No. 20020015439 teaches a 11-half-chip based reacquisition technique. U.S. patent application No. 20020169550 teaches a reacquisition technique using a supplementary internal guidance system. U.S. patent application No. 20030118086 teaches reacquisition using dual correlators having distinct correlation times.
Accordingly, there is a need in the art for an efficient reacquisition technique, which avoids the false lock on the auto-correlation side-lobes or on the frequency side-lobes.